Purification of alkali metal nitrates

The process for removing heavy metal contaminants from an impure alkali metal nitrate containing them, said impure alkali metal nitrate being comprised of a mixture of between about 30 and 70 percent by weight of sodium nitrate and between about 30 and about 70 percent by weight of potassium nitrate, wherein said process comprises:

• a. admixing said impure alkali metal nitrate with sufficient water to form a concentrated aqueous solution thereof containing from between about 20 and about 70 percent by weight of alkali metal nitrate;
• b. adjusting the pH of said concentrated aqueous solution to within the range from between 2 and about 7;
• c. admixing sufficient reducing agent with said concentrated aqueous solution of alkali metal nitrate to provide at least the stoichiometric proportion necessary to react with heavy metal impurities contained therein, wherein said reducing agent is selected from the group consisting of alkali metal sulfide, alkali metal bisulfite, alkali metal borohydride, and hydrazine;
• d. admixing sufficient base with the aqueous alkali metal nitrate solution containing said reducing agent to raise the pH of the resulting mixture to above about 6, whereby a precipitate of solid heavy metal contaminants forms within said aqueous solution;
• and
• e. separating said solid heavy metal contaminants from the resulting purified aqueous solution of alkali metal nitrate.